As electronic devices become more and more complex, the need for greater and greater numbers of transistors on the device is increased. In addition, power consumption needs to be reduced while the speed of the devices needs to be increased. At least part of the answer to these requirements involves reducing the area that each transistor occupies. However, this may adversely affect one or more of the other requirements. More specifically, as the transistors are scaled down, the gate structure is also scaled down and this increases the resistance of the gate. Hence, the power consumption is increased and the speed of the device is decreased.
Several attempts to reduce the sheet resistivity of the gate structures have been made in the past. First, the polycrystalline silicon was more heavily doped with either n-type or p-type dopants. Then, the upper portion of the gate was silicided with tungsten or titanium. Presently, cobalt silicide is being used so as to reduce the resistivity for smaller geometries. The next likely solution will involve metal gate structures.
Metal gate structures provide lower sheet resistivity virtually irrespective of the width of the gate. However, many metal gate materials have problems which must be overcome before they can be implemented in a standard semiconductor processing flow. One problem is that many metals are unstable next to SiO.sub.2, which is commonly used for the gate dielectric layer. Another problem is that many metals become less conductive when they are oxidized.
Aluminum and tungsten have been used to form gate structures. Aluminum may not be a good choice due to the problems stated above and tungsten has a work function which lies between the work function of p-type polycrystalline silicon (poly) and n-type poly. A problem with tungsten, though, is that as the applied voltages become smaller and smaller the fact that the work function is midgap and is unchangeable (as compared to n-type and p-type poly), it may become difficult to provide a gate potential greater than the threshold voltage of the PMOS and NMOS device.
In an attempt to over come this threshold voltage problem using one midgap metal for both PMOS and NMOS device, aluminum has been utilized for one type of devices while platinum is used for the other type of devices. However, platinum is expensive and difficult to work with and aluminum suffers from the problems listed above. Hence, a need exists for a gate electrode material whose conductivity is not relative to the gate width and which has different work functions for PMOS devices and NMOS devices.